1. Field of the Invention
The present invention relates to a method of measuring a thickness and an etching depth in which the etching amount of a processed material is detected by emission spectroscopy in the manufacturing of a semiconductor integrated circuit or the like, and particularly relates to a method and apparatus for measuring the depth and thickness of a processed material, by which the amount of etching of each layer formed on a substrate by etching using plasma discharge is correctly measured and a desired thickness and etching depth are suitably obtained, and a method and apparatus for processing a processed material using the same.
2. Description of the Related Art
In the manufacturing of semiconductor wafers, dry etching is widely used for removing layers made of various materials, particularly layers of dielectric materials formed on wafer surfaces, or used for forming patterns. In the control of process parameters, it is the most important to accurately determine an etching endpoint for stopping etching at a desired thickness t and etching depth during the processing of these layers.
During the dry etching of a semiconductor wafer, emission intensity at a specific wavelength in plasma light changes with the progress of etching of a specific film. As a method of detecting the etching endpoint of a semiconductor wafer, the following conventional method is available: a change in emission intensity at a specific wavelength is detected from plasma during dry etching and the etching endpoint of a specific film is detected based on the detection result. In this case, it is necessary to prevent erroneous detection caused by a detection waveform which fluctuates with noise. A method of accurately detecting a change in emission intensity includes a detecting method conforming to the method of moving average (for example, refer to Japanese Patent Laid-Open No. 61-53728 (Patent Document 1)) and a method of reducing noise by primary approximation according to the method of least squares (for example, refer to Japanese Patent Laid-Open No. 63-200533 (Patent Document 2)).
As the recent designs of semiconductors become finer with higher densities and higher integration, an aperture ratio (the etched area of a semiconductor wafer) decreases and emission intensity becomes weak at a specific wavelength captured from an optical sensor to a photodetector. As a result, the level of a sampling signal from the photodetector decreases and it is difficult for an endpoint decision unit to positively detect the endpoint of etching based on the sampling signal from the photodetector.
When detecting the endpoint of etching and stopping processing, actually it is important that the remaining thickness of a dielectric layer is equal to a predetermined value. In the conventional process, the overall process is monitored according to time-thickness control technique predicated on a constant etching speed of each layer. The etching speed is determined by, for example, processing a sample wafer beforehand. In this method, the etching process is stopped according to time supervision after the passage of time corresponding to a predetermined etching thickness.
However, it is known that an actual film, for example, an SiO2 film formed by low pressure chemical vapor deposition (LPCVD) has a less reproducible thickness. The tolerance of a thickness in process fluctuation of LPCVD corresponds to about 10% of the initial thickness of the SiO2 film. Therefore, the actual final thickness of the SiO2 film remaining on a silicon substrate cannot be correctly measured by the method of time supervision. The actual thickness of the remaining layer is finally measured by a technique using a standard spectral interferometer. When excessive etching is found, the wafer is discarded as a reject.
Further, it is known that an insulating film etching device degrades with time. For example, the etching speed is reduced by repeated etching and etching may be stopped halfway. Such a problem has to be solved. In addition, it is also important to monitor the time variations of the etching speed to achieve stable process operation. In the conventional method, only time is monitored to decide an endpoint and no proper method is available for the time variations and fluctuations of the etching speed. Moreover, when deciding an end point for a short etching time of 10 seconds, it is necessary to shorten preparation time for decision and sufficiently reduce segments of decision time, which has not been sufficiently achieved in the conventional method. In many cases, an insulating film has an etched area of 1% or less and a small change in the intensity of plasma radiation from a reaction product generated by etching. Therefore, an endpoint decision system capable of detecting a small change is necessary but no practical and inexpensive system is available.
In other methods, the etching endpoint of a semiconductor wafer is detected using an interferometer. In a first method, interference light (plasma light) is detected using color filters of red, green and blue to detect the endpoint of etching (for example, refer to Japanese Patent Laid-Open No. 5-179467 (Patent Document 3)). In a second method, the extreme values of an interference waveform (the maximum and minimum points of the waveform and 0 point of a differential waveform) are counted using the time variations of the interference waveform of two wavelengths and the differential waveform. An etching speed is calculated by measuring a time until the count reaches a predetermined value, the remaining etching time until a predetermined thickness is determined based on the calculated etching speed, and the etching process is stopped according to the etching time (for example, refer to Japanese Patent Laid-Open No. 8-274082 (U.S. Pat. No. 5,658,418) (Patent Document 4)). In a third method, a difference waveform is determined (using a wavelength as a parameter) between a light intensity pattern of interference light before processing (using a wavelength as a parameter) and a light intensity pattern of interference light after processing or during processing, and a height difference (thickness) is measured by comparing the difference waveform and a difference waveform in a database (for example, refer to Japanese Patent Laid-Open No. 2000-97648 (Patent Document 5)). A fourth method relates to a rotary coating applicator. In this method, the time variations of interference light of multiple wavelengths are measured to determine a thickness (for example, refer to Japanese Patent Laid-Open No. 2000-106356 (Patent Document 6)). In a fifth method, the characteristic time variations of interference light are determined by measurement and stored in a database, the end of etching is decided by comparing the database and a measured interference waveform, and the change of etching process conditions is accelerated by the decision (for example, refer to U.S. Pat. No. 6,081,334 (Patent Document 7)).
In the methods using interferometers, monochromatic radiation from a laser is incident at a vertical incidence angle on a wafer including a laminated structure of different materials. For example, on a wafer where an SiO2 layer is stacked on an Si3N4 layer, interference fringes are formed by radiation reflected on the top surface of the SiO2 layer and radiation reflected on the boundary surface between the SiO2 layer and the Si3N4 layer. The reflected radiation is emitted to a proper detector and generates a signal which is varied in thickness with the thickness of the SiO2 layer during etching. When the top surface of the SiO2 layer is exposed during an etching process, the etching speed and the current etching thickness can be correctly monitored in a continuous manner without delay. In some methods, predetermined plasma radiation is measured instead of laser radiation by spectrometers.
The conventional techniques cause the following problems:
A. When a thickness is decided in a thickness processing process (resist etchback or the like with a thickness of several μ), the time variations of interference light become complicated to several periods or more, and thus even a small disturbance affects the decision.B. When a thickness is decided in a thickness processing process (etchback or the like of a gate oxide film or an oxide film), it is necessary to measure a small change of interference light and a small disturbance affects the decision. In other words, the time variations of interference light are ½ to ¼ periods or less during the processing of a thin film, interference fringes slightly change, and the influence of noise has to be eliminated to decide a thickness.C. On a processed wafer for mass production, peripheral circuits are mixed and various materials (mask material, etched material, and other materials on the peripheral circuits) are etched at the same time. Thus, interference light from different materials are superimposed in a complicated manner, the various materials are varied in thickness in a lot or between lots of processed wafers, and the time variations of interference light are changed in a lot or between lots during etching.D. When producing small batches of a variety of products, various etching processes are mixed and thus an etching device is likely to degrade with time and cause abnormal discharge and change of plasma. For this reason, plasma radiation changes and disturbance is superimposed on interference light, which affects a decision.
In consideration of these points, it has been difficult to correctly measure and control the remaining amount or etching depth of a processed layer with a desired measurement accuracy, particularly a processed layer in plasma etching.
It is an object of the present invention to provide an etching endpoint decision method using a method of measuring the thickness or etching depth of a processed material, a plasma processing apparatus for implementing the endpoint decision method, by which the actual remaining amount or etching depth of the processed layer can be correctly measured during plasma etching in a process of manufacturing a semiconductor device, and a plasma processing method and apparatus using the same.
Another object of the present invention is to provide an etching method which makes it possible to correctly control each layer of a semiconductor device online to a predetermined thickness and a predetermined etching depth in a process of manufacturing the semiconductor device.
Still another object of the present invention is to provide a device for measuring the thickness or etching depth of a processed material whereby the actual thickness and etching depth of a processed layer can be correctly measured online in a process of manufacturing a semiconductor device.